A high strength galvanized steel sheet for use as an automobile component or the like is required to have excellent workability as well as high strength, due to the nature of its application.
Recently, as a vehicle body becomes increasingly lightweight in view of improving fuel efficiency, a high strength steel sheet is required to be used in an automobile body to ensure collision safety, and the scope of application of such a high strength steel sheet is expanding. Further, although a high strength steel sheet is normally subjected to light shaping in the conventional technique, it is now being studied to apply the high strength steel sheet to a complex shape.
However, the workability of a steel sheet generally deteriorates as the strength thereof increases. Accordingly, when a high strength steel sheet is applied to a vehicle body, there arises a problem such as fracturing when the steel sheet is subjected to press forming. In particular, the aforementioned problem is likely to occur in applying a high strength steel sheet having a tensile strength of 980 MPa or larger to a component that is to be bent/formed.
Further, since vehicle body processing involves an assembly process subsequent to press forming and it is necessary to perform resistance spot welding in the assembly process, excellent weldability is also required along with workability.
To meet the above-mentioned requirements, for example, JP-A 2004-232011, JP-A 2002-256386, JP-A 2002-317245, JP-A 2005-105367, JP-B 3263143, JP-B 3596316, and JP-A 2001-011538 each propose a method of obtaining a high strength galvanized steel sheet with high workability by specifying steel components and structures or optimizing hot rolling conditions and annealing conditions. Further, JP-A 02-175839, JP-A 05-195149, JP-A 10-130782, JP-A 2005-273002, and JP-A 2002-161336 each disclose a technology of obtaining a cold-rolled steel sheet excellent in bendability, JP-A 2006-161064 discloses a technology of obtaining a high tensile strength galvanized steel sheet excellent in bendability, and JP-A 2008-280608 discloses a technology of providing a galvanized steel sheet excellent in workability and weldability.
Among the above-cited publications, JP-A 2004-232011 describes a steel satisfying a tensile strength of 980 MPa or so and having a high content of C and Si. However, JP-A 2004-232011 gives no consideration to improvement in stretch flangeability and bendability.
Similarly, JP-A 2002-256386, JP-A 2002-317245 and JP-A 2005-105367 each disclose a steel member that utilizes Cr, but gives no consideration to improvement in stretch flangeability and bendability.
JP-B 3263143, JP-B 3596316, and JP-A 2001-011538 each refer to ductility ratio λ, which is one of the indexes for evaluating stretch flangeability. However, the tensile strengths (TS) of steel sheets, subjected to the measurement of the ductility ratio, of these publications fail to reach 980 MPa. Further, these publications are silent about bendability of the steel sheets thereof.
JP-A 02-175839, JP-A 05-195149, JP-A 10-130782, JP-A 2005-273002 each disclose a technology involving bendability of steel sheets by softening the steel sheets in regions measured from surfaces thereof in the thickness direction to a depth of at least 10 vol % or at least 10 μm. However, there arises a problem in that the soft layer in the steel sheet surface layer is so thick that fatigue strength deteriorates.
JP-A 2002-161336 describes that a steel sheet can be improved in bendability when a soft layer is formed in the steel sheet surface to a depth up to 10 μm. However, JP-A 2002-161336 fails to specify the steel sheet structure and thus inevitably faces a problem that fatigue strength of the steel sheet decreases as a whole.
JP-A 2006-161064, although it discloses improving bendability of a steel sheet by setting the area ratio of a ferrite phase to be 80% or more in vicinities of the top surfaces (a depth of 1 to 10 μm measured from the surface layer) of the steel sheet, fails to make any reference to the internal structure of the steel sheet. Further, JP-A 2006-161064 has no description of weldability and plane-bending fatigue properties of the steel sheet, thereby still having a problem in terms of weldability and plane-bending fatigue properties.
JP-A 2008-280608 discloses a high strength galvanized steel sheet excellent in workability and weldability, as well as bendability to attain a critical bend radius not larger than 1.5t (hereinafter, t represents sheet thickness of the steel sheet) at 90° V-bending. JP-A 2008-280608 has actually achieved a critical bend radius as small as 0.36t. However, to further expand the scope of application of the high strength steel sheet to a vehicle body, bendability needs to be further increased, that is, critical bend radius needs to be further reduced. Specifically, critical bend radius not larger than 0.3t is required.
It could therefore be helpful to provide a high strength galvanized steel sheet having very good bendability and weldability, which specifically has high tensile strength of 980 MPa or larger and satisfies a durability ratio, defined as fatigue limit/tensile strength, of 0.35 or larger without deteriorating the plane-bending fatigue property, as well as an advantageous production method of the steel sheet.